An outline of a continuous casting equipment is illustrated in FIG. 19. A molten steel prepared by a steel converter and secondary refining is put into a ladle 51, and poured into a mold 4 through a tundish 52. The molten steel which is in contact with the mold 4 is cooled and solidified, transported by rolls 54 while a casting speed thereof is controlled, and cut into a proper length by a gas cutting machine 55. In the continuous casting of steel as stated above, there is a possibility that a fluid state and a solidified state of the molten steel in the mold 4 incur a casting stop due to a deterioration trouble of properties of a cast slab. It is therefore necessary to estimate and control the state in the mold by online to enable stable casting and to manufacture a cast slab without defect.
A cross section of the continuous casting equipment in a vicinity of a mold is illustrated in FIG. 20. A reference numeral 1 is molten steel, a reference numeral 2 is a solidified shell, a reference numeral 3 is a mold flux layer, a reference numeral 4 is a mold, a reference numeral 5 is cooling water, and a reference numeral 8 is an immersion nozzle.
As illustrated in FIG. 20, the molten steel 1 is poured from the immersion nozzle 8 into the mold 4, and a cast slab whose side surface is solidified is pulled out of a bottom of the mold 4 in a process of the continuous casting. There are unsolidified parts in the cast slab in a vicinity of a lower end of the mold 4, and they are entirely solidified at a secondary cooling part at a lower layer than the mold 4.
In an operation of the continuous casting, high-speed casting is aimed to enable improvement in productivity. However, when the casting speed is too fast, the solidified shell 2 being the cast slab which is solidified at the side surface of the mold 4 is pulled outside the mold 4 with insufficient strength, and an operation trouble called as a break-out is incurred because the solidified shell 2 is broken and the molten steel 1 outflows in the continuous casting equipment in an extreme case. Once the break-out occurs, the operation is stopped to perform removal of the steel which outflows and is solidified in the equipment and repair of the equipment, as a result, a lot of time is required to recover the operation, and there is a large loss.
There are proposed various casting technologies such as development of a high-speed casting powder, improvement in a cooling mechanism of a mold copper plate, and a temperature management to enable a stable high-speed casting without generating the operation trouble such as the break-out (Non-Patent Literature 1).
Besides, there is also proposed a technology in which soundness of a solidified shell in a mold is diagnosed from measurement values of mold temperatures or the like, a casting state is determined whether or not it leads to a break-out to control a casting speed or the like by using the determination result. For example, in Patent Literature 1, there is proposed a detection technology of a restrictive break-out. In this example, the restrictive break-out is avoided by measuring temperatures by thermocouples embedded in a mold, capturing a time-series change of characteristic thermocouple temperatures observed when a shell fracture occurs resulting that the solidified shell is restricted to the mold, recognizing a fracture surface of the solidified shell in the mold, and decreasing a casting speed before the fracture surface reaches a lower end of the mold.
However, the break-out is not limited to the restrictive one, and there are ones each of whose sign of the break-out is difficult to appear in a temperature waveform representing the time-series change of the temperature. One of them is a break-out due to drift. The break-out due to drift is a break-out which occurs when unexpected circumstances such as drift of a molten steel flow in the mold 4 or the like occur, a heat quantity over cooling capacity of the mold 4 is locally applied to the solidified shell 2 to inhibit a solidification growth, and the solidified shell 2 with insufficient strength is pulled outside the mold 4. In the continuous casting, the molten steel 1 is poured from the immersion nozzle 8 into the mold 4, but there is a case when the break-out due to drift is induced when erosion of the immersion nozzle 8 occurs, a discharge port excessively deforms caused by generated inclusions, for example, during casting. It is difficult to directly observe a drift phenomenon, and characteristics thereof are difficult to appear also in the mold temperature waveform different from the restrictive break-out.
As a detection technology of the break-out due to drift as stated above, there are proposed development of technologies such that it becomes possible to estimate a state in a mold owing to an inverse problem method where other information such as the casting speed and a cooling water temperature are taken into account in addition to the mold temperature, and the occurrence of the break-out is prevented as described in Patent Literatures 2 to 5. In Patent Literature 2, there is described the inverse problem method estimating the solidified state in the continuous casting. Besides, in Patent Literatures 3 to 5, there is described a method controlling casting to avoid an operation trouble by using estimation amounts representing the state in the mold obtained by the method according to Patent Literature 2. However, in Patent Literatures 3 to 5, there are proposed a method to determine an abnormal casting state leading to the break-out and an avoidance method, but they are not generalized, and a concrete method to determine allowable limit values to determine the abnormal casting is not specified. Accordingly, when the technologies described in Patent Literatures 3 to 5 are actually used, it is often the case to rely on an experience of an executant. Besides, there is not referred to cases when there are differences in variations of estimation results depending on casting conditions, and therefore, there is a possibility that excessively low allowable limit values are set.
Besides, there is also proposed a technology estimating a heat flux from temperatures measured at a plurality of points in a mold by using a heat transfer inverse problem method to detect the break-out (Patent Literature 6).